1. Field of the Invention
The present invention relates to a branching filter, which is particularly suitable for use in conjunction with telecommunications apparatus such as a radio transmitter and/or receiver apparatus for separating or multiplexing signals having different frequencies in accordance with the frequency.
2. Description of the Prior Art
Such a type of branching filters are known in Japanese patent laid-open publication Nos. 136104/1987 and 136105/1987, for example. See also U.S. Pat. No. 5,015,973 entitled "Duplexer with An Isolating Circuit On A Dielectric Plate" to Kawakami, Komazaki, Gunji, Onishi, Sakurai, Horii, and Mashimo, many of whom are co-inventors herein. FIGS. 3A and 3B show structures of such a type of conventional branching filter. FIG. 3A is a perspective view of the conventional branching filter, and FIG. 3B is a bottom perspective view of the same. This branching filter is provided with an insulative substrate 1 such as alumina or glass-epoxy resin, on a front side and a back side of which substrate there are formed a plurality of input/output terminals 3 and grounding conductor patterns 2 by means of thick-film printing, plating processing and the like. The plurality of input/output terminals 3, which are provided on the front side and the back side of the substrate 1, are formed in pairs, and each pair of terminals are coupled to each other by a through hole for connection between the front side and the back side. Referring to FIG. 3a, on the front side of the substrate 1, there are directly mounted transmitter dielectric filter 6 and receiving dielectric filter 7, which are mutually different in a central frequency.
In FIG. 3a, filters 6 and 7 are, as described in Japanese patent laid-open publication No. 80901/1986, for example, provided with a block configuration of filter main bodies 4 and 5 each consisting of unitary homogeneous dielectric, a plurality of dielectric resonators 8 and 12 each consisting of a cylindrical configuration of central conductor embedded at regular intervals within the filter main bodies 4 and 5, and a plurality of frequency regulation patterns 8 and 13 each connected to associated one of the central conductors of the dielectric resonators 8 and 12, formed in one side of the filter main bodies 4 and 5, respectively. At the both ends of the filter 6, there are provided input/output terminals 3 and input/output electrodes 14 and 15 connected with the through holes, and at the both ends of the filter 7, there are provided input/output terminals 3 and input/output electrodes 10 and 11 connected with the through holes. A resonance frequency of each of the dielectric resonators is determined by the height of a dielectric resonator and a frequency regulation pattern, and the regulation of the resonance frequency may be performed by mechanical or optical technical skill.
On the back side of the substrate 1 in FIG. 3b, there are formed a pair of branching or splitting filter circuits 16 and 17 comprising distributed constant lines such as strip lines, and a lowpass filter 18 for spurious suppression, by means of thick-film printing, plating processing and the like. The branching filter circuits 16 and 17 are connected through the input/output terminals 3 and the input/output electrodes 10, 11, 14 and 15 to the filters 6 and 7 on the front side, respectively.
In order to avoid mutual influence between the branching filter circuit 16 and the receiver filter 7 connected in series, and the branching filter circuit 17 and the transmitter filter 6 connected in series, it is needed to provide a sufficiently high input impedance for the branching filter circuit 17 and the transmitter filter 6, at a pass band central frequency of the branching filter circuit 16 and the receiver filter 7. Thus, the line length of each of the branching filter circuits 16 and 17 is determined as follows.
Now considering an application of a cascade connection of the branching filter circuits 16 and the receiver filter 7. In this case, and as discussed in the aforementioned Kawakami et al. U.S. Patent, a component S.sub.11 (1) of an S dispersion matrix is given by the following equation (1): ##EQU1## wherein .theta.=.beta.l, .beta.=2.pi./.lambda., l=line length of the branching filter circuit 17, .lambda. is the wavelength of the receiver frequency, and r+j z=input impedance for the transmitting filter with r as the real part of the input impedance of the transmitter filter, z is the constant term of the imaginary part and j is the imaginary unit.
In order to provide a sufficiently high input impedance for the branching or multiplexing filter circuit 17 and the transmitter filter 6, at a pass band central frequency of the receiver filter 7, it is sufficient for component S.sub.11 (1) of the equation (1) to be minimized. In other words, it is understood that parameter .theta. may be selected to satisfy the following equation (2). EQU cos .theta.=z sin .theta. (2)
Substituting this into Equation (1) leads to Equation (3), as follows: EQU S.sub.11 (1)=[rcos.theta.+j(sin.theta.+z.sup.2 sin.theta.-rsin.theta.)]/[rcos.theta.+j(sin.theta.+z.sup.2 sin.theta.+rsin.theta.)] (3)
Now, if the equation (3) is expressed by the input impedance (Zin), then EQU Zin=[(1+z.sup.2)/r]-j z (4)
That is, it is needed to provide a sufficiently high input impedance for the branching filter circuit 17 and the transmitter filter 6 connected in series, as a pass band of the branching filter circuit 16 and the receiver filter 7 connected in series.
Thus, the branching filter circuit 17 and the transmitting filter 6 connected in series form an attenuator. In this case, it may be considered in the equation (4) that parameter r is sufficiently smaller than unity, that is, r.ltoreq.1. Consequently, in the equation (4). Zin.gtoreq.1, thus mutual influence may be avoided. It would be understood that in order to provide a sufficient large real part in the equation (4), an phase angle of Zin has to be zero.
However, according to the structure of the branching filter as mentioned above, the line length of the distributed constant line becomes approximately .lambda./4, and thus this becomes one of the critical drawbacks to miniaturization of the branching filter and achieving low-cost.
For example, take the case where a conductor having width 1.8 mm is formed on a glass-epoxy resin having thickness 1 mm (dielectric constant 4.8) so as to provide input impedance 50 .OMEGA.. The resulting relation between the line length and the phase angle is shown in FIG. 4. As apparent from FIG. 4, in order to obtain the phase angle 0.degree., a line length of 60 mm at the receiving side and a line length of 84 mm at the transmitting side are needed. Thus it would be difficult to achieve miniaturization of the filter and reduction in cost.
A branching filter by which the foregoing problem has been solved is disclosed in Japanese patent laid-open publication No. 60004/1989 (see also U.S. Pat. No. 5,015,973). FIGS. 5A and 5B hereof show structures of the branching filter disclosed in the JP laid-open publication. FIG. 5A is a perspective view of such a branching filter, and FIG. 5B is a bottom perspective view of the same. In those figures, reference numeral 20 denotes a substrate such as glass-epoxy resin; 21 a ground conductor pattern; 22 an antenna terminal; 23 and 25 terminals of Rx (receiver filter); 24 and 26 terminals of Tx (transmitter filter). Referring to FIG. 5a, 27 denotes a transmitter filter main body; 28 a receiver filter main body; 29 a transmitter filter; 30 a receiver filter; 31 a dielectric resonator of the receiver filter; 32 a frequency regulator pattern of the receiver filter; 33 a coupling amount regulator pattern of the receiver filter; 34 and 35 input/output terminals of the receiver filter; 36 a dielectric resonator of the transmitter filter; 37 a frequency regulator pattern of the transmitter filter; 38 a coupling amount regulator pattern of the transmitter filter; and 39 and 40 input/output terminals of the transmitter filter.
On the back side of the substrate 20 in FIG. 5b, there are formed inductors each comprising a fine line conductor. More specifically, reference numerals 41, 42 and 43 are directed to inductors L.sub.AR 41, L.sub.RE 42 and L.sub.RT 43, respectively, instead of a distributed constant circuit of the separating or branching filter circuit. Numeral 44 represents an exposure portion of the substrate.
The branching filter shown in FIGS. 5A and 5B is, for instance, a branching filter for U.S. AMPS (Advanced Mobile Phone Service) scheme land mobile radiotelephone use, which comprises a transmitter filter (N.sub.1) 29 designed for a central frequency (f.sub.o) of 835 MHz and a pass band (BW) of 825-845 MHz, a receiver filter (N.sub.2) 30 designed with f.sub.o of 880 MHz and BW of 870-890 MHz, and a branching filter circuit.
FIG. 6 is a block diagram of the branching filter shown in FIGS. 5A and 5B (FIG. 6 corresponds generally to FIG. 2 of U.S. Pat. No. 5,015,973.). The mounted filters (N.sub.1) 29 and (N.sub.2) 30 are provided with a block configuration of filter main bodies 27 and 28 each consisting of unitary homogeneous dielectric, a plurality of dielectric resonators 31 and 36 each consisting of a cylindrical configuration of central conductor embedded at regular intervals within the filter main bodies 27 and 28, a plurality of frequency regulation patterns 32 and 37 each connected to the associated one of the central conductors of the dielectric resonators 31 and 36, formed in one side of the filter main bodies 27 and 28, and coupling amount regulator patterns 33 and 38 between associated ones of the frequency regulator patterns 32 and 37, respectively as seen in FIG. 5a. The dielectric resonators 31 and 36 located on both the sides of the individual filters are provided with input/output patterns 34, 35, 39 and 40, which are connected to input/output terminals 23, 25, 26 and 24, respectively as seen in FIG. 5b. The several terminals of FIGS. 5A and 5B are shown in the FIG. 6 schematic diagram.
FIG. 7 is a graph plotting input impedance characteristics (absolute value) of the transmitter filter N.sub.1, in which a frequency (MHz) is given on a horizontal axis, and an absolute value of input impedance (.OMEGA.) is given on a vertical axis, In FIG. 7, there are shown two cases of input/output end capacitances 2.3 pF and 2.5 pF. As apparent from FIG. 7, the input impedance is approximately 50 .OMEGA. in the range of 825-845 MHz which is the pass band of transmitter filter N.sub.1, and rapidly increases when the frequency exceeds such a pass band.
FIG. 8 plots input impedance characteristics (phase angle) of the transmitting filter N.sub.1, in which frequency (MHz) is given on a horizontal axis, and phase angle .theta. (.omega.).degree. is given on a vertical axis. In FIG. 8, there are shown two cases of input/output end capacitances 2.3 pF and 2.5 pF. As apparent from FIG. 8, the phase angle is approximately 0 at 825-845 MHz which is the pass band of the transmitting filter N.sub.1.
FIG. 9 shows input impedance characteristics (absolute value) of the receiving filter N.sub.2, in which frequency (MHz) is given on a horizontal axis, and an absolute value of input impedance (.OMEGA.) is given on a vertical axis. In FIG. 9, there are plotted three curves of input/output end capacitances 1.8 pF, 2.0 pF and 2.2 pF. As clear from FIG. 9, the input impedance is approximately 50 .OMEGA. at 870-890 MHz which is the pass band of the receiver filter N.sub.2.
FIG. 10 is a view in which input impedance characteristics (phase angle) of the receiver filter N.sub.2 are plotted with frequency (MHz) given on its horizontal axis and phase angle .theta. (.omega.).degree. given on its vertical axis. In that figure, there are plotted three curves of input/output end capacitances 1.8 pF, 2.0 pF and 2.2 pF. As apparent from the figure, the phase angle is approximately 0 at 870-890 MHz which is the pass band of the receiving filter N.sub.2.
It is thus understood that the branching or multiplexer filter as mentioned above is, as shown in FIGS. 5A, 5B and 6, provided with the inductors L.sub.AR 41, L.sub.RE 42 and L.sub.RT 43 instead of the distributed constant circuit.
A principle of the branching filter as mentioned above will be described hereinafter on the basis of the operation of those inductors. First, let us consider an instance of no inductors L.sub.AR 41, L.sub.RE 42 and L.sub.RT 43. As a matter of convenience in description, there will be stated only the central frequencies f.sub.o =835 (MHz), f.sub.o =880 (MHz) of the respective pass bands (B.sub.1, B.sub.2) of the transmitter and receiver filters N.sub.1 and N.sub.2.
Regarding B.sub.1 [f.sub.o =835 (MHz)] and B.sub.2 [f.sub.o = 880 (MHz)], they are considered from FIGS. 7-10 as follows: ##EQU2## where r.sub.1 is the real part of the input impedance of the transmitter filter when the input impedance is expressed as r+jz, x.sub.1 is the constant term of the imaginary part thereof, r.sub.2 is the real part of the input impedance of the receiver filter when the input impedance is expressed as r+jz and x.sub.2 is the constant term of the imaginary part thereof
If the branching filter is constituted by using those transmitting and receiving filters, then input impedance Zin and mismatch accentuation amount or return loss RL, seeing from antenna terminal 22, are given as follows: ##EQU3## From the foregoing, it is apparent that the input impedance Zin and the Return Loss result in an undesirable reduction compared with a case of unity of the transmitting filter N.sub.1 or the receiving filter N.sub.2. Further, it is a problem that X.sub.2 is small at the band B.sub.1. Under such a condition that x.sub.2 is small, the inductor L.sub.RT =5 (nH) is connected in series to the transmitter filter N.sub.1 so that the input impedance Zin at the band B.sub.1 may approach a reference impedance (R.sub.o =50 .OMEGA.). Input impedance Z.sub.in and mismatch attenuation amount RL, at that time, are given as follows: ##EQU4##
Next, the inductor L.sub.RE will be described hereinafter. If there is provided the inductor L.sub.RE of 20 nH, that is, L.sub.RE =20 nH, then input impedance Zin and mismatch attenuation amount RL, at B.sub.1 and B.sub.2 after insertion of the inductor L.sub.RE, are given as follows: ##EQU5##
Hence, insertion of the inductor L.sub.RE serves to provide a relatively small imaginary part of Z.sub.in in comparison with a real part of Zin at the respective bands B.sub.1 and B.sub.2.
Next, the inductor L.sub.AR will be described. If there is provided the inductor L.sub.AR of 4 nH, that is, L.sub.AR =4 nH, input impedance Zin and mismatch attenuation amount RL, at B.sub.1 and B.sub.2 after insertion of the inductor L.sub.AR, are given as follows: ##EQU6##
Hence, insertion of the inductor L.sub.AR serves to provide a small imaginary part of Zin on an average basis at the respective bands B.sub.1 and B.sub.2. Thus, it is possible to obtain the branching filter satisfying RL&gt;10 dB necessary for the land mobile radiotelephone. While those inductors L.sub.AR, L.sub.RE and L.sub.RT were explained by way of example, it is possible to expect a similar operation as far as the tendency of Zin of the those filters N.sub.1 and N.sub.2 is not changed.
Where the inductors L.sub.AR, L.sub.RE and L.sub.RT are formed on a glass-epoxy resin substrate having the dielectric constant of 4.8 and the thickness of 1.6 mm, the inductance is given by the following relationships, where f.sub.o =850 (MHz), l=line length:
(1) In case of W (width)=0.3 mm: EQU L(nH)=1.389l (mm)-5.3443 PA1 In case of L.sub.RT =5 nH, l=7.45 mm PA1 In case of L.sub.RE =20 nH, l=18.25 mm PA1 In case of L.sub.AR =4 nH, l=6.73 mm PA1 (2) In case of W (width)=0.5 mm: EQU L(nH)=1.092l (mm)-2.4726 PA1 (3) In case of W (width)=0.7 mm: EQU L(nH)=1.0135l (mm)-2.1753 PA1 (1) According to the branching filter shown in FIG. 3, the branching filter circuit is constructed on a .lambda./4 line basis. This structure causes the line of the branching filter circuit to relatively extend, and thus to enlarge the occupied area. Consequently, it is difficult to provide miniaturization of the filter and low-cost. PA1 (2) According to the branching filter shown in FIG. 5A and 5B, the branching filter circuit is constructed on the basis of the inductor by the strip line so that the entire line length of the strip line can be shortened. However, that is limited in its shortening, so that the branching filter circuit may occupy some extent of area. This becomes drawbacks to miniaturization of the branching filter and achieving low-cost.
Thus, for instance, in cases of the above-mentioned inductors L.sub.AR, L.sub.RE and L.sub.RT, there are obtained the following cases of line length:
If the inductors are formed on a glass-epoxy resin substrate having the dielectric constant of 9.3, it is possible to further reduce the line length l.
Those conventional branching filters as mentioned above have, however, been associated with the following drawbacks.